Why is Pluto emitting X-rays?

The detection of X-rays coming from Pluto is challenging scientists to understand more about the space surrounding the best-known object in the outer solar system.

While NASA’s New Horizons spacecraft was speeding toward and beyond Pluto, by NASA’s Chandra X-ray Observatory—in orbit back at Earth—was aimed several times on the dwarf planet and its moons, gathering data that the missions could compare. Each time Chandra pointed at Pluto—four times in all, from February 2014 through August 2015—it detected X-rays coming from the small planet.

The first detection of Pluto in X-rays was made using the Chandra telescope in conjunction with observations from the New Horizons spacecraft, which flew by the dwarf planet in 2015. (Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute/CXC)

That was somewhat surprising, given that Pluto—cold, rocky and without a magnetic field—has no natural mechanism for emitting X-rays.

“We’ve just detected, for the first time, X-rays coming from an object in our Kuiper Belt, and learned that Pluto is interacting with the solar wind in an unexpected and energetic fashion,” says Carey Lisse, an astrophysicist at Johns Hopkins University Applied Physics Laboratory, who led the Chandra observation team with APL’s New Horizons co-investigator Ralph McNutt. “We can expect other large Kuiper Belt objects to be doing the same.”

Pluto is the largest object in the Kuiper Belt, a vast population of small, distant bodies orbiting the sun. The belt extends from the orbit of Neptune, 30 times the distance of Earth from the sun, to about 50 times the Earth-sun distance.

Lisse, who first detected X-rays from a comet two decades ago, knew that X-rays from Pluto—though not likely—were possible. The interaction between gases surrounding planetary bodies and the solar wind—the constant streams of charged particles speeding out from the sun—can create X-rays, which are high-energy electromagnetic waves with a very short wavelength.

New Horizons scientists wanted to learn about the interaction between Pluto’s atmosphere and the solar wind. The spacecraft carries an instrument designed to measure that activity up-close, the aptly named Solar Wind around Pluto. Scientists are using SWAP data to craft a picture of Pluto with a very mild, close-in bow shock, where the solar wind first “meets” Pluto (similar to a shock wave that forms ahead of a supersonic aircraft) and a small wake behind the planet. The immediate mystery is that Chandra’s readings on the brightness of the X-rays are much higher than would be expected from the solar wind interacting with Pluto’s atmosphere.

“Before our observations, scientists thought it was highly unlikely that we’d detect X-rays from Pluto, causing a strong debate as to whether Chandra should observe it at all,” says coauthor Scott Wolk of the Harvard-Smithsonian Center for Astrophysics. “Prior to Pluto, the most distant solar system body with detected X-ray emission was Saturn’s rings and disk.”

Although Pluto is releasing enough gas from its surprisingly stable atmosphere to make the observed X-rays, in simple models for the intensity of the solar wind at the distance of Pluto, there isn’t enough solar wind flowing directly at Pluto to make them.

Researchers suggest several possibilities for the enhanced X-ray emission from Pluto. One is that there is a much wider and longer tail of gas trailing Pluto than New Horizons found with SWAP. Another is that interplanetary magnetic fields are focusing more particles than expected from the solar wind into the region around Pluto. A third is that the low density of the solar wind in the outer solar system could allow formation of a doughnut, or torus, of neutral gas centered on Pluto’s orbit.

That the Chandra measurements don’t quite match up with New Horizons up-close observations is the benefit—and beauty—of an opportunity like the New Horizons flyby.

“When you have a chance at a once in a lifetime flyby like New Horizons at Pluto, you want to point every piece of glass—every telescope on and around Earth—at the target,” McNutt says. “The measurements come together and give you a much more complete picture you couldn’t get at any other time, from anywhere else.”

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